read a file of specific content in erlang - functional-programming

Question

I followed this method to read files - how to read the contents of a file In Erlang?
The answer mentioned works good. However, my file content looks like this:
{0, {data, node, 0}}
{1, {someData, node, 1}}
Every line will have this data. So while reading file I want to filter on how many lines it should read
something like this:
read(Node, FirstIndex, LastIndex, F, Acc)
Here, F is the fold function, which takes a single log entry and the current accumulator and returns the new accumulator value.
How can I incorporate the solution with this function that I need??

Answer 1

-module(my).
-compile(export_all).
get_section(Start, End, Fname) when Start>0, End>=Start ->
{ok, Io} = file:open(Fname, read),
ok = advance_file_pointer(Start, Io),
Lines = getNLines(End-Start+1, Io),
file:close(Io),
Lines.
advance_file_pointer(1, _Io) ->
ok;
advance_file_pointer(Start, Io) ->
{ok, _Line} = file:read_line(Io),
advance_file_pointer(Start-1, Io).
getNLines(N, Io) ->
getNLines(N, Io, []).
getNLines(0, _Io, Acc) ->
lists:reverse(Acc);
getNLines(N, Io, Acc) ->
{ok, Line} = file:read_line(Io),
getNLines(N-1, Io, [Line|Acc]).
In the shell:
~/erlang_programs$ cat data.txt
{1, {data, node, 1}}
{2, {someData, node, 2}}
{3, {otherData, node, 3}}
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.2 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:get_section(1, 1, "data.txt").
["{1, {data, node, 1}}\n"]
3> my:get_section(1, 2, "data.txt").
["{1, {data, node, 1}}\n","{2, {someData, node, 2}}\n"]
4> my:get_section(1, 3, "data.txt").
["{1, {data, node, 1}}\n","{2, {someData, node, 2}}\n",
"{3, {otherData, node, 3}}\n"]
5> my:get_section(2, 3, "data.txt").
["{2, {someData, node, 2}}\n","{3, {otherData, node, 3}}\n"]
6> my:get_section(2, 2, "data.txt").
["{2, {someData, node, 2}}\n"]
7> my:get_section(0, 2, "data.txt").
** exception error: no function clause matching my:get_section(0,2,"data.txt") (my.erl, line 4)
8> my:get_section(1, 4, "data.txt").
** exception error: no match of right hand side value eof
in function my:getNLines/3 (my.erl, line 20)
in call from my:get_section/3 (my.erl, line 7)
9>
You can apply your fold function in the second clause of getNLines/3.